A fairly simple idea to add a twist to a Bookhounds of London campaign is to make one of the Player Characters works for an intelligence agency. They might be a detective in the Metropolitan Police’s Special Branch working under cover, a freelance agent recruited by MI5, an agent of another organisation turned by MI5 or even an MI5 Case Officer based outside of the Office.
The shop the PCs work for may be a regular shop, it may be an MI5 front used for dead drops and meetings with agents, another agencies dead drop that MI5 has discovered or a shop known to be popular with radicals, anarchists, neo-fascists, communists, artists or all sorts of other disreputable sorts that need to be kept an eye on. There might be a safe house above the shop or in the basement.
Things to Consider
One important question the GM should consider – does the owner know or suspect? If they do are they a part of the organisation, an unwilling participant or an unwitting accomplice. If they don’t know what will they do if they find out. If the owner is a PC the revelation that another PC is working for MI5 may be more problematic than if the owner is an NPC.
If you want more than one PC (or shop related NPCs) to be in on the MI5 connection other characters might be babysitters protecting the shop, a bridge agent who carries messages back to the Office or a Cobbler who works better away from the Office.
Another important consideration is to not allow the addition of more conventional PCs than bookhounds change the core difference that a bookhounds campaign has from a regular Trail of Cthulhu campaign. To achieve that the GM should force MI5 characters to work largely outside of MI5. Their superiors should be unwilling or unable to supply resources and cash. They may dismiss the supernatural. They may task the PC with jobs that distract from a current situation. Occasionally they may be able to help out with information, a last-minute rescue or conveniently covering up a cult.